Foundation manual on power measurement

One correspondent was confident that the Foundation candidates are properly trained, which leads to examining the training materials.

Above is an extract of the WIA’s “Your entry into amateur radio”, the training manual for the Australian Foundation licence. Click on it for a larger view.

Dealing with the text…

Some SWR meters also have a power meter calibrated in watts.

That is true, and modern instruments fall into two main styles:

those that use an RF voltmeter to drive the meter calibrated in watts; and

those that provide switch selectable calibration of the directional couplers to read forward and reflected power directly.

The one that is pictured is neither type, it is a very poor design that provides a calibration chart to ‘calibrate’ the forward meter in watts over the meter’s frequency range by adjustment of the sensitivity control according to the chart. The system is quite inaccurate because of difficulty in setting the sensitivity knob accurately, and in any event, only one coupler is calibrated meaning it must use a 50Ω resistive load for measurement.

This measurement is only accurate when the SWR is exactly 1:1, and the load impedance is exactly 50 ohms.

That statement, whilst true of type 1 meters mentioned above, and the example shown, is not true in the general case, not true of the type 2 instruments which are very common these days. It is a classic ham myth that has worked its way into the training manual for the next generation of hams. In fact, if a directional wattmeter is calibrated for any real impedance (which is the usual case, eg 50+j0Ω , 75+j0Ω), power is given by Pfwd-Pref (irrespective of whether the instrument is used in a line of nominal characteristic impedance equal to its chalibration impedance). I gave the mathematical proof of this in articles on VK1OD.net (Duffy 2007), (Duffy 2012), (Duffy 2013).

To comply with the conditions of the licence, the power meter must be set to peak power.

The term is Peak Envelope Power (PEP) or pX in ITU terms, the LCD uses pX.

Unfortunately, many directional wattmeters in the ham market pretend to indicate Peak Envelope Power on SSB telephony, but they can be proven abject failures by putting them in tandem with a reputable instrument (eg Bird 43 with PEP). As a broad guideline, if the meter does not use an active amplifier, does not require a battery or external power to make the PEP function work, it probably doesn’t accurately indicate PEP on SSB telephony. There are plenty of meters that do work properly, and plenty that don’t. Meters that do not work properly, invariably underestimate the true PEP and so encourage breach of the applicable power limit.

Of course, you will note that the example meter shown does not have a “peak power” switch, it cannot properly indicate PEP on SSB telephony and the use of it as discussed at Foundation watts explained can lead to gross breaches of the power limit, even if unintended.

You must adjust your transmitter’s power to no more than 10 watts peak power to conform to the conditions of the Foundation Licence.

This whole section has not given technically sound guidance on instrumentation appropriate to the task, or technically sound method of measuring PEP and adjustment of the transmitter (eg controls to be used) for compliance.

In that environment, it is likely that there will be significant non-compliance.

It is my experience that most hams do not understand the concepts of peak envelope power, nor its measurement in a practical setting so it is even more challenging for entry level licencees. It is not that measuring PEP is so difficult given good advice and appropriate test equipment, it is getting good advice that is the problem.

References

Duffy, O. 2007. VSWR measurement. VK1OD.net (offline).

———. 2012. Power in a mismatched transmission line. VK1OD.net (offline).